1. Field of the Invention
This invention relates to hermetic, vertical axis rotary helical screw compressors, and more particularly, to such hermetic compressors as disclosed in U.S. Pat. No. 3,922,114 issued Nov. 25, 1975, and assigned to the common assignee.
2. Description of the Prior Art
Hermetic, vertical axis, rotary helical screw compressors have evolved, particularly in low horsepower size, as unitary pieces of equipment including within the hermetic housing, means for separating and cooling the oil which is necessary for lubrication of the moving parts and for forming compression chambers or closed threads between the intermeshed helical screw rotors and the surrounding compressor housing. Further, by incorporating within such vertical axis hermetic rotary helical screw compressor packages, the electrical drive motor which is open to the compressor discharge, the motor windings may be readily cooled, that is, maintained at a relatively low operating temperature by the discharge gas as it moves vertically upward from the compressor which underlies the electric drive motor, prior to discharge of the compressed gas at the upper end of the hermetic outer enclosure.
In the hermetic, rotary helical screw compressor of U.S. Pat. No. 3,922,114, the outer, generally cylindrical and sealed enclosure supports internally a two part inner cylindrical housing which forms with the outer enclosure a sealed chamber, while the inner housing itself forms a high pressure or discharge side second chamber, housing the electric drive motor and a third low pressure or inlet gas chamber sealed from the first and second chambers and within which is incorporated the helical screw rotors defining the compression chambers. In that hermetic compressor design, the helical screw rotors and the compressor drive motor rotor are supported for rotation about their axes by sleeve bearings or the like. An oil pump fixed to one of the compressor helical screw rotor shafts and being driven thereby acts to pressurize the oil accumulating within the bottom of the outer container which acts as a sump for the oil. Such oil under high pressure is conducted through passages within the screw rotor shafts for lubricating the sleeve bearings and the intermeshed helical screw rotors. Further, in that compressor design, the suction side of the screw compressor unit is at the lower end of the vertical assembly, the high pressure gas discharges vertically upwards, at the upper end of the meshed helical screw rotors and is directed through suitable passages to the second chamber, housing the electric motor. Necessarily, due to the reaction forces developed during compression of the gas, balance pistons connected to the rotors require applied oil pressure on the side opposite the discharge gas to balance the developed thrust forces. In order to separate the oil entrained in the discharge gas by way of the compression process and employed for lubricating the bearings and for sealing the intermeshed rotary helical screw rotors, the hermetic design of U.S. Pat. No. 3,922,114 includes an incorporated oil separation scheme. Axial passages are provided within the electric motor rotor to effect centrifugal oil separation and through the electric motor stator for draining separated oil and radial, preferably canted, discharge passages extend through the hermetic inner casing upper chamber wall into the first chamber defined by the outer enclosure for tangential impingement of the separated oil on the outer enclosure wall.
While the vertical, hermetic rotary screw compressor of that design operates satisfactorily, the structural make up is fairly complicated, expensive to manufacture and requires overthrust compensation for the developed axial thrust forces to insure minimum gap dimension between the helical screw rotors on the discharge side of the compressor and fixed end plates defining the second and third chambers which house the electric drive motor and the helical screw rotors respectively.
It is therefore an object of the present invention to provide an improved, vertical axis, rotary helical screw compressor of the hermetic type which eliminates the necessity for balance pistons and the necessity for an oil pump without compromising the wear life of the rotary bearings of the hermetic compressor.
It is a further objection of the present invention to provide an improved vertical, hermetic rotary helical screw compressor in which some oil introduced to the compressor working fluid is distributed to the compressor and drive motor bearings during normal passage of compressor discharge gas to the discharge outlet of the hermetic unit.
It is a further object of the present invention to provide an improved vertical, hermetic rotary helical screw compressor in which oil separation is achieved in a simplified manner without the need of internal and external sealed housings.